The separation of fluids, either in gaseous or liquid form, has become increasingly important, especially in fields involving the purification of liquids. The process for the separation of liquids involves the utilization of a reverse osmosis system such as the purification of water and especially saline water or the removal of impurities from fluids such as blood in the field of dialysis. When utilizing reverse osmosis in the purification of water such as saline water, a pressure in excess of the osmotic pressure of the saline water feed solution is applied to the solution which is prepared from purified water by a semipermeable membrane. The purified water thereby diffuses through the membrane while the sodium chloride molecules or other impurities which may be present in the water are retained by the membrane. Various semipermeable membranes are now being used in commercial processes for performing separations by the reverse osmosis treatment of aqueous solutions either for a portion of relatively pure water or for concentration of a liquid solution being treated, or both. An example of the semipermeable membranes which are used will include the early Loeb-type membranes which are fashioned from cellulose diacetate by the process such as described in U.S. Pat. Nos. 3,133,132 and 3,133,137. These Loeb-type membranes comprise an asymmetric type which are characterized by a very thin, dense surface layer or skin which is supported upon an integrally attached, much thicker supporting layer. In addition to this type of semipermeable membrane, other types of membranes which are in use will include those which have been fabricated from polyamides, polyimide, polyphenyl esters, polysulfonamides, polybenzimidazole, polyarylene oxides, polyvinylmethyl ether and other polymeric organic materials.
In addition to the reverse osmosis system which is employed for the separation of liquids, it is also possible to separate various gases which are present in a gaseous feed mixture.
It is taught in U.S. Pat. No. 4,243,701 to Riley et al that certain membranes may also be utilized for the separation of various gases. The separation of a gas mixture utilizing a membrane is effected by passing a feed stream of the gas across the surface of the membrane. Inasmuch as the feed stream is at an elevated pressure relative to the effluent stream, a more permeable component of the mixture will pass through the membrane at a more rapid rate than will a less permeable component. Therefore, the permeate stream which passes through the membrane is enriched in the more permeable component while, conversely, the residue stream is enriched in the less permeable component of the feed.
Another type of separation process in which various components of fluid mixtures are separated involves the use of adsorbents such as molecular sieves. In the adsorption type of process, the adsorption exhibits a selectivity for one component of the mixture over another or, with a molecular sieve, one component is retained more than other components. The adsorbent may be employed in the form of a dense compact fixed bed which is alternatively contacted with the feed mixture and desorbent materials. In one embodiment, the adsorbent is employed in the form of a single static bed in which case the process is only semicontinuous. In another embodiment, a feed of two or more static beds may be employed in a fixed bed contact with appropriate valving employed in the flow scheme so that the feed mixture is passed through one or more adsorbent beds while the desorbent material is passed through one or more of the other beds in the operation. The flow of said mixture and desorbent material may be effected in either an upward or downward flow through the adsorbent. The most commercially successful embodiment of the adsorptive type separation process comprises the countercurrent moving bed or simulated moving bed countercurrent flow scheme. In such a type of system the adsorption and desorption operations are continuously taking place which allows both continuous production of an extract and raffinate stream along with the continual use feed and desorbent streams.
Various types of materials which may be employed as separation membranes have been shown in prior U.S. patents. For example, the incorporation of two components in a membrane system has been shown in U.S. Pat. Nos. 3,457,180, 3,878,104, 3,993,566, 4,032,454 and 4,341,605 which teach the use of structural supports or reinforcement fibers or fabrics to aid the membrane in resisting the high pressures which are utilized in a reverse osmosis process. U.S. Pat. No. 3,556,305 discloses a "sandwich" type reverse osmosis membrane comprising a porous substrate covered by a barrier layer which, in turn, is covered by a polymer or film bonded to the barrier layer by an adhesive polymeric layer. U.S. Pat. No. 3,862,030 discloses a polymeric matrix having an inorganic film such as silica dispersed throughout the matrix to impart a network of microvoids or pores of a size of about 0.01 to about 100 microns, which are capable of filtering microscopic or ultrafine particles of submicron size. U.S. Pat. No. 4,302,334 discloses a membrane "alloy" comprising a hydrophobic fluorocarbon polymer blended with a polyvinyl alcohol polymer which imparts hydrophilic properties to the membrane.
U.S. Pat. No. 4,230,463 discloses a multicomponent membrane which may be useful for the separation of gases comprising a polymer coating on a porous separation membrane, the latter may also comprise a polymer such as polysulfone. However, the polysulfone support which is used to prepare this membrane is not unduly porous in nature and possesses a large ratio of total surface area to total pore cross-sectional area. The patent particularly discloses the use of membranes having minimum ratios of total surface area to total pore cross-sectional area of about 1000:1. In addition, the patent discusses the separation of nonpolar gases as a primary function of the membrane, not the coating.
Other types of membranes which may be employed to effect the separation of gases may comprise the mixed matrix type of membrane such as molecular sieves incorporated with polymeric membranes. One particular type of mixed matrix membrane comprises a type 5A (Linde) zeolite incorporated with a silicon rubber matrix. This type of membrane was disclosed in an article "The Diffusion Time Lag in Polymer Membranes Containing Adsorptive Fillers" in J. Polymer Sci.; Symposium #41, 79-93 (1973). This article teaches that the zeolite "filler" causes a time lag in reaching steady state permeation of the membrane by various gases due to the adsorption of the gases by the zeolite. It is taught in this article that once the zeolite becomes saturated by the permeate gas, a steady state rate of permeation through the membrane is reached so that the membrane selectivity is essentially the same as if the zeolite was not present.
As will hereinafter be shown in greater detail, we have now discovered that a multicomponent membrane may be utilized to effect a separation of polar gases from nonpolar gases in a process which results in the obtention of a high flux in combination with a high separation factor.